Electrode/current collector, laminates for an electrochemical device

ABSTRACT

The invention relates to a cheap and fast method for the production of electrode/current collector laminates, the electrode layer having a thickness of at least 20 μm. The current collector foil is provided with an electrode paste coating on at least one of its sides by filling paste into the cells or grooves of a patterned matrice roll; transferring paste from the matrice roll onto the current collector foil by a printing operation involving contacting the matrice roll and the current collector foil or alternatively contacting the matrice roll with an offset roll which in turn is in contact with the current collector foil; drying the layer of paste printed onto the current collector; and optionally repeating the process until the desired thickness of the electrode layer is obtained. The invention also relates to a gravure roll and a primer-coated current collector foil for use in the method, a primer paste for the production of primer-coated current collector foils and an electrode paste for the production of electrode/current collector laminates.

The present invention relates to a method for the production ofelectrode/current collector laminates for an electrochemical device, anelectrode paste, a gravure roll and a primer-coated current collectorfoil for use in the method, a primer paste for the production ofprimer-coated current collector foils, an electrode paste for theproduction of electrode/current collector laminates and a gravure roll.The invention furthermore relates to the electrode/current collectorlaminates produced.

Traditionally, components for electrochemical cells have been bulky andhave been produced by mould casting, pressing, punching and rollingtechniques.

The increasing demand for high capacity and high power density batterieshas resulted in the development of laminar batteries. Such batteries aregenerally produced by laminating together thin layers of currentcollectors, electrodes and optionally a mechanical separator.Electrode/current collector components for laminar cells areconventionally produced by pressing, extrusion, pasting or doctor bladecoating techniques (see e.g. “Handbook of Batteries”, McGraw-Hill,1995).

U.S. Pat. No. 4,935,317 describes a process for making a solid statelaminar cell involving doctor blade coating of a cathode compositionlayer on a current collector, followed by rolling of the externalsurface of said cathode composition to provide an external surfacehaving minimal surface discontinuities.

EP 411 949 describes a method for the formation of a thin layer oflithium metal onto a substrate by transferring molten metal, which isprojected above the surface of a vessel, to a substrate by directly orindirectly contacting the molten metal with the surface of the metal.

For the production of the thin cell components used in laminarelectrochemical systems like lithium polymer batteries andsupercapacitors, however, none of the above-mentioned methods areappropriate, since they do not allow for continuous high speedproduction of components of uniform thickness.

The thickness of the components of laminar electrochemical devices is inthe range of 5-200 μm. Such laminar components should be of uniformthickness, since non-uniformity may lead to a non-uniform resistancedistribution over the laminate area, non-uniform current distributionand eventually hot spots and cell failure. Accordingly, these devicescall for a manufacturing method which allows the application of layersof uniform thickness. In particular, a need exists for a method for theproduction of electrode/current collector laminates which allows forapplication of an electrode layer of uniform thickness on a currentcollector despite the presence of substrate irregularities.

Rechargeable electrochemical cells, such as lithium polymer batteries,may be manufactured in their charged as well as their discharged state.However, due to the fact that the electrodes of such cells are generallyhighly reactive in their charged state, charged-state production callsfor production conditions involving inert atmosphere, addingsignificantly to the production costs. Thus, production ofelectrochemical cells in their discharged state, which do not call forsuch precautions, is generally desirable.

The electrode/electrolyte interface of laminar electrochemical cellstypically consists of a structure including electrode components as wellas electrolyte components, intimately mixed to high percolation. Thisinterface structure may be obtained by an in-situ one-step operation, inwhich a paste containing electrode as well as electrolyte components isapplied to a current collector. Due to the hygroscopicity of non-aqueouselectrolytes, this operation must be performed in an inert atmosphere.

Following an alternative route of manufacture, however, electrolyte-freeelectrode/current collector laminates may be produced, which aresubsequently impregnated with electrolyte. In this case the part of themanufacturing process, which does not involve the electrolyte, can takeplace in ambient atmosphere. Electrodes manufactured in this way shoulddisplay controlled porosity as to allow the subsequent absorption of theelectrolyte.

Therefore, although a number of techniques are known for the manufactureof laminar components for electrochemical systems, there still exist aneed for a technique, which fulfils the requirements for fast,reproducible manufacture of electrode/current collector laminates ofuniform thickness, despite the presence of irregularity of the surfacesof the current collector foil.

It is thus an object of the invention to provide an efficient andeconomically advantageous method for continuous high speed production ofelectrode/current collector laminates having an electrode layer ofuniform thickness despite the presence of current collector foilirregularities, the thickness of the electrode layer being at least 20μm.

This object is accomplished by a method for the production of anelectrode/current collector laminate for an electrochemical device, saidelectrode layer of the laminate having a thickness of at least 20 μm,wherein said current collector foil of the laminate, optionally coatedwith a conductive primer, is provided with an electrode paste coating onat least one of its sides, said electrode paste comprises particles ofelectrode material, a solvent and a binder, said method comprises thestep of:

a) filling said paste into the cells or grooves of a patterned matriceroll;

b) transferring paste from the matrice roll onto the current collectorfoil by a printing operation involving contacting the matrice roll andthe current collector foil or alternatively contacting the matrice rollwith an offset roll which in turn is in contact with the currentcollector foil;

c) drying the layer of paste printed onto the current collector; and

d) optionally repeating steps a)-c) until the desired thickness of theelectrode layer is obtained.

In the present context the expression “electrochemical devices” i.a.encompass batteries, including rechargeable batteries, preferablylithium-polymer batteries, fuel cells, capacitors, including so-calledsupercapacitors, electromechanical reactors and electrochromic deviceshere under electrochromic displays, including “smart windows”.

Electrochromic displays, which are also referred to as variabletransmission windows or smart windows, are transparent electrochromicsystems, the colour of which can be controlled upon variation of theapplied potential. Those electrochromic displays comprise a workingglass electrode, traditionally comprising WO₃, which is bleached uponcharge and coloured upon discharge and a counter glass electrode. Bothof these electrodes are coated onto a transparent current collectorlayer, traditionally of ITO glass, and sandwiched between saidelectrodes, is the electrolyte.

It is preferred that the steps a)-c) of the method according to theinvention is performed using gravure printing technique.

The gravure printing technique involves transfer of a paste of pigmentand/or printing ink from the engraved cells of a gravure roll onto aweb, alternatively onto a second roll and subsequently onto a web(offset gravure printing). Traditionally, the conditions of a gravureprinting operation involves coating of pastes having a viscosity of15-1500 mPas at a printing speed of up to 300 m/min. The virtuallynon-porous coatings obtained typically have a thickness in the range of1-10 μm.

The gravure printing technique is basically a loading procedure, inwhich a fixed amount of material is applied on a substrate. The generalconditions and measures to be taken to perform gravure printing are e.g.described in Cohen, Edward and Gutoff, Edgar: Modern Coating and DryingTechnology, VCH, pp. 103-108 and Leach and Pierce: The printing inkmanual, Chapmann & Hall, 1993, pp. 474-546.

Application of gravure printing in other types of battery and capacitormanufacturing processes has been described in the literature:

U.S. Pat. No. 5,227,223 describes the gravure printing of metalliccatalytic inks having a very low viscosity (20-50 cp. in the examples)comprising a solution of polymer and a group IB or VIII metal onto amoving web, for the production of electronically conductive componentsfor devices like diodes, capacitors, batteries, sensors and fuel cells.The metallic catalytic ink is printed onto the web in fine lines of awidth of e.g. less than 100 μm and a thickness being less than 8 μm (Thedepth of the image in the gravure roll is less than 80 μm and thecontent of solids in the ink is not more than 10% by weight).

U.S. Pat. No. 2,688,649 describes the printing of batteries using an inkwith suspended magnetic particles, printing said ink in thin lines ontoa non-conducting sheet for subsequent orientation of the magneticparticles of the ink in a magnetic field. It is not directly mentionedthat the thickness of the layer of magnetic particles is thin, howeverfrom the drawings it appears that the magnetic particles is lying in onelayer. In other words it may be concluded the thickness of the printedink is close to the maximal particle size.

U.S. Pat. No. 3,006,980 describes the patterns or laterally spaced thinlinear anodes printed onto one side of an electrically non-conductivesheet, and the patterns of parallel, laterally spaced thin linearcathodes printed onto the other side of said electrically non-conductivesheet, the anodes and cathodes, respectively, being connected by aprinted linear electrically conductive transverse thin trunk line.

None of these above prior art documents, however, gives any teachingwhich could lead a skilled person to the solution of the presentinvention. Particularly it should be observed, that nothing is mentionedabout how to provide a conducting collector foil with an electrode layerof a thickness of at least 20 μm which layer substantially having auniform thickness through out the layer. Also it should be observed thatsuch electrode layers should cover most of at least one side of thecurrent collector optionally leaving one or more edge parts of thecurrent collector foil free of electrode material. The electrode layerpreferably covers an area of at least 5×10 mm of the collector foil.

Bierwagen evaluated in Electrochemica Acta Vol. 37 (1992) pp. 1471-8 theuse of a number of coating techniques like roll coating, gravurecoating, dip coating, bead coating, curtain coating, slide coating,spray coating and spin coating for their potential use in themanufacture of solid polymer batteries. However, Bierwagen was onlyconcerned with the application of these techniques for the coating ofsolid polymeric electrolyte (SPE) films.

The pattern of the matrice roll may be either uniform or non-uniform. Inthe present context a uniform pattern is represented by an essentiallyhomogeneous and coherent pattern of cells or grooves on the rollsurface. A non-uniform pattern, on the other hand, is represented by anon-homogeneous and/or discontinuous pattern. Thus, a roll having on itssurface e.g. two or more engraved domains separated by non-engravedparts, has a non-uniform pattern according to this definition.

The matrice roll is preferably an engraved gravure roll. In this casethe electrode paste can be transferred onto the current collector foileither by a direct gravure printing operation or an offset gravureprinting operation. The direct gravure printing operation involvesintroduction of the current collector foil into the nip between a pairof rolls consisting of the engraved gravure roll and a flexible back-uproll. The offset gravure printing operation involves transfer of pastefrom the gravure roll to an offset roll in contact with the gravure rolland subsequently transfer of the paste from the offset roll to thecurrent collector foil by introduction of the current collector foilinto the nip between a pair of rolls consisting of said offset roll anda flexible back-up roll.

Alternatively, the electrode paste can be transferred onto the currentcollector foil by a flexoprinting operation. Flexoprinting is performedby employing a patterned matrice roll and a flexible rubber roll (theoffset roll) having a rastered surface. More specifically, theflexoprinting operation involves transfer of paste from the patternedroll to the rubber roll and subsequently transfer of the paste from therubber roll to the current collector foil by introduction of the currentcollector foil into the nip between a pair of rolls consisting of saidrubber roll and a flexible back-up roll.

The thickness of the electrode layer in the produced electrode/currentcollector layer should be relatively high (higher than 20 μm) in orderto ensure high electrode capacity and thus high energy and power densityof the final electrochemical device. The electrode paste is preferablycoated onto the current collector in an amount sufficient to obtain anelectrode layer thickness of 20-300 μm in the produced laminate.

This electrode layer thickness may be obtained using a single electrodepaste printing operation. Alternatively, one or more, preferably 1-5,subsequent electrode coating operations, in which electrode paste isprinted onto the surface of an electrode layer of a previously producedelectrode/current collector laminate, followed by drying of the printedelectrode paste, may be employed.

The porosity of the produced electrode layer should preferably berelatively high so as to allow for efficient impregnation of theelectrode layer with an electrolyte solution. The electrode paste ispreferably coated under such conditions as to obtain a porosity of20-80%, more preferably 30-70%, in the electrode layer of the producedlaminate.

When one or more subsequent electrode coating operations are performed,in which electrode paste is printed onto the surface of a previouslydeposited electrode layer, the paste composition and the employed dryingconditions may be different from layer to layer. In this way electrodesdisplaying a porosity profile (i.e. a varying porosity throughout thedepth of electrode) may be produced in a simple manner. The porosityprofile can be obtained by coating one or more subsequent electrodelayers on top of the first electrode coating, at least one of which hasa porosity which differs from the first electrode coating. However, manyother electrode porosity profiles can easily be obtained.Advantageously, electrodes displaying an increasing or decreasingporosity as a function of the depth of the electrode may be produced. Ina preferred embodiment, electrodes showing a relatively low porositynear the current collector and a relatively high porosity near the uppersurface are produced.

The current collector foil should consist of a material which fulfilsthe demands for high chemical and electrochemical stability, highelectronical conductivity, low density and good processability.Furthermore it should provide a good adhesion to layers coated thereon.

According to the invention the current collector foil is preferably ametal foil or a foil of a conductive polymer. Aluminium, copper, nickeland stainless steel are all available as thin foils, displaying a highconductivity. Thus, foils of aluminium, copper, nickel and stainlesssteel are the most preferred current collectors according to theinvention.

In batteries, copper and nickel foils are preferably used as currentcollectors at the negative electrodes, as they generally are not stableagainst oxidation at the operational potential of positive electrodes.Aluminium, on the other hand, is preferably used as a current collectorat the positive electrode, as aluminium may form alloys with themetalions of the electrolyte salt, e.g. Li⁺, at low potentials. Alloyingnot only leads to a loss of active material and loss of potential; theformed alloys, such as lithium-aluminium alloys, are generally porousand of low density, leading to irreversible volume changes and failureof the negative electrode.

In order to alleviate the above-mentioned disadvantages normallyassociated with current collector foils and to protect the currentcollector foils from highly reactive and corrosive electrode andelectrolyte materials, they are advantageously coated with a dense,conductive primer, prior to the coating of electrode paste. The primermay be based on commercially available products, such as the CL51 screenprinting dye from Wiederhold Siebdruckfarben, which are based on carbonblack/graphite and thermosettling resins. However, during the course ofthe research underlying the present invention a particularlyadvantageous primer has been developed.

Accordingly, the present invention furthermore relates to a primer pastefor the production of current collectors having a protective primerlayer, said primer paste being composed essentially of

3-30% by weight, preferably 5-20% by weight, of carbon blacks;

0-20% by weight, preferably 0-10% by weight, of graphite;

6-40% by weight, preferably 10-25% by weight, of binder;

40-90% by weight, preferably 50-80% by weight, of solvent; and

0-10% by weight, preferably 1-5% by weight, of auxiliary materials,preferably selected from the group consisting of dispersing agents,defoaming agents and rheological control agents.

The carbon blacks and graphites are preferably selected from highlyconductive carbon blacks, preferably having a conductivity of at least 1S.cm⁻, and graphites having a low lithium intercalation capability and alower reactivity toward the electrolyte of the electrochemical devicethey are intended for.

Furthermore, the carbon blacks preferably display high structure and areadvantageously selected from the group consisting of furnace blacks,acetylene blacks and lampblacks. The graphites preferably displayparticle size of 10 μm or less, advantageously in the range of 0.5-5 μm,preferably 0.5-2.5 μm.

The binder is advantageously selected from the group consisting ofrubber binders, preferably ethylene-propylene-diene-polymethylene(EPDM), ethylene-propylene-polymethylene (EPM) and polyisobutylene(PIB), cellulosederivatives, preferably nitrocellulose (NC), celluloseacetate butyrate (CAB) and cellulose acetate propionate (CAP), resins,preferably indene-coumarone resins and aromatic C₉-hydrocarbon resins,fluorocontaining polymers, preferably PVDF and PVDF-copolymers,including PVDF-PE copolymer and PVDF-hexafluoro propylene copolymers,vinyl copolymers, modified vinyl .copolymers, polyacrylates, polyamides,polyur-ethanes, polyisocyanates and blocked polyisocyanates reacted withhydroxylbearing polymers or di- or triamines.

The solvent is preferably selected from the group consisting of C₅-C₁₅aliphatic or alicyclic compounds, organic esters of the general formulaR¹C(═O)OR², wherein R¹ represents hydrogen or C₁-C₄ alkyl and R²represents C₁-C₅ alkyl, glycol derivatives of the general formulaR³OCHR⁴CH₂OR⁵, wherein R³ and R⁵ independently represent hydrogen, C₁-C₄alkyl or acetyl and R⁴ represents hydrogen or methyl, alcohols R⁶—OH,wherein R⁶ represents C₁-C₄ alkyl, ketones, DMF, N-methyl-pyrrolidone,dimethylacetamid, DMSO and THF.

The dispersing agent may be non-ionic, anionic, cationic, as well asamphoteric. However, preferably a cationic dispersing agent, such asDeuteron PO100 from W.O.C. Schöner GmbH, is used.

The defoaming agent is preferably a mineral oil or silicone oildefoaming agent, preferably a silicone oil defoaming agent, such asBYK-080 from BYK Chemie.

The rheological control agent is preferably selected from organo clays,silicas and castor oil derivatives, preferably organo clays, such asViscogel B7 form Chimica Mineraria SpA.

The primer layer may be applied to the current collector using a widevariety of coating/printing techniques. The coating technique used ispreferably selected from the group consisting of doctor blade coating,wire bar coating, screen printing, direct gravure printing, offsetgravure printing and flexoprinting, more preferably by direct gravureprinting or screen printing, even more preferably by direct gravureprinting.

The primer layer preferably has a thickness of 1-10 μm, more preferably1-5 μm.

It may be desirable to produce electrode/current collector laminatesconsisting of a current collector foil onto which electrode layers arecoated on both sides of the foil. Such laminates are easily produced bythe method of the invention, as the production thereof merely involves afirst step in which a first electrode paste is coated onto a first sideof the current collector, followed by drying, and a second step in whicha second electrode paste is coated onto the opposite side of the currentcollector, followed by drying, this second electrode paste beingidentical to or different from the first electrode paste.

The present invention furthermore relates to an electrode paste for usein the method of the invention. This electrode paste comprises anelectrode material, binder, solvent and optionally an electronicallyconductive additive.

The electrode material may consist of any material capable offunctioning as the electrochemically active ingredient of a positive ornegative electrode of an electrochemical device. When used as anelectrode material in a rechargeable battery, it should be capable ofbeing oxidized and reduced in a reversible manner during charge anddischarge of the battery. Examples of suitably electrode materials forthe production of batteries are:

Positive Electrodes

lithium transition metal oxides, preferably lithium manganese oxides,more preferably spinel lithium manganese oxide, vanadium oxides,preferably V₆O₁₃ or VO₂, lithium cobalt oxides, preferably LiCoO₂, andlithium nickel oxides, preferably LiNiO₂.

Negative Electrodes

Li-intercalating carbon materials, preferably cokes, carbon blacks andgraphites.

The relative amount of electrode material in the paste has a majorinfluence on the energy and power density achieveable from theelectrochemical device in which the produced electrode/current collectorlaminate is used. In general, a low amount of electrode material resultsin a low energy density. On the other hand, a high amount of electrodematerial may result in a low power density, owing to reduced accessthereto as caused by low porosity and/or low electronical conductivityof the produced electrode layer. The electrode material is preferablypresent in an amount of 10-60% by weight, more preferably 25-50% byweight, relative to the total weight of the electrode paste.

The electrode paste preferably contains an electronically conductiveadditive, such as carbon black, in order to enhance electrodeconductivity and thus the device power density. This additive ispreferably present in an amount of 0-20% by weight, more preferably2-10% by weight.

The binder loading should be sufficiently high to provide an electrodelayer displaying high mechanical integrity during manufacture as well asin the final product. On the other hand it should also be sufficientlylow to allow for a high electrode porosity. The binder is preferablypresent in an amount of 1-20% by weight, preferably 1-10% by weight,relative to the total weight of the electrode paste.

The binder is advantageously selected from the group consisting ofrubber binders, preferably ethylene-propylene-diene-polymethylene(EPDM), ethylene-propylene-polymethylene (EPM) and polyisobutylene(PIB), cellulosederivatives, preferably nitrocellulose (NC), celluloseacetate butyrate (CAB) and cellulose acetate propionate (CAP), resins,preferably indene-coumarone resins and aromatic C₉-hydrocarbon resins,fluorocontaining polymers, preferably PVDF and PVDF-copolymers,including PVDF-PE copolymer and PVDF-hexafluoro propylene copolymers,vinyl copolymers modified vinyl copolymers, polyacrylates, polyamides,polyur-ethanes, polyisocyanates and blocked polyisocyanates reacted withhydroxylbearing polymers or di- or triamines.

The paste solvent is preferably a low boiling solvent, which is easilyevaporated at relatively low drying temperature during manufacture. Onthe other hand, too fast evaporation may lead to mud cracks and poorcoatings. The solvent should preferably have a high solvating powertowards the binder, allowing production of highly concentrated pastes.

The solvent is preferably present in an amount of 20-88% by weight, morepreferably 30-70% by weight, relative to the total weight of theelectrode paste, and is advantageously selected from the groupconsisting of C₆-C₁₅ aliphatic or alicyclic compounds, organic esters ofthe general formula R¹C(═O)OR², wherein R¹ represents hydrogen or C₁-C₄alkyl and R² represents C₁-C₅ alkyl, glycol derivatives of the generalformula R³OCHR⁴CH₂OR⁵, wherein R³ and R⁵ independently representhydrogen, C₁-C₄ alkyl or acetyl and R⁴ represents hydrogen or methyl,alcohols R⁶—OH, wherein R⁶ represents C₁-C₄ alkyl, ketones, DMF,N-methyl-pyrrolidone, dimethylacetamid, DMSO and THF.

The viscosity of the electrode paste should be relatively low in orderto allow for a high electrode coating thickness and to obtain goodlevelling properties of the coated paste. The viscosity of the paste ata shear rate of 700 s⁻¹ is preferably in the range of 50-1000 mPas.Furthermore, the paste preferably displays pseudoplasticity, i.e. aviscosity which decreases as a function of the shear rate. The paste mayeven display a thixotropic viscosity profile.

The electrode paste may furthermore contain certain auxiliary materials,such as dispersing agents, defoaming agents and/or rheological controlagents, in an amount of 0-20% weight, preferably 1-10% by weight, basedon the total weight of the electrode paste. The purpose of suchauxiliary materials is to ensure a suitably low paste viscosity despitethe relative high loading of electrode material and binder, so as toallow for efficient filling of gravure cells and proper subsequentpick-out of electrode paste, whilst at low shear rates, the pasteviscosity remains sufficiently high so as to provide for good adhesionbetween electrode paste and gravure roll and electrode paste and currentcollector.

Generally, the presence of dispersing agents and defoaming agentsinfluence the viscosity level at all shear rates and does not cause anysignificant changes of the curvature of the viscosity profile. Incontrast, rheological control agents have a major influence on thecurvature of the viscosity profile, and may accordingly be used toadjust the pseudoplastic and/or thixotropic properties of the electrodepaste.

Examples of suitable dispersing agents are non-ionic, anionic, cationic,such as Deuteron PO100 from W.O.C. Schöner GmbH, and amphoteric, such asTroysol 98C from Troy Chemical Company, dispersing agents. Preferably acationic dispersing agents is used.

Examples of suitable defoaming agents are mineral oil and silicone oildefoaming agents, preferably silicone oil defoaming agents, such asBYK-080 from BYK Chemie.

Examples of suitable rheological control agents are organo clays,silicas and castor oil derivatives, preferably organo clays, such asViscogel B7 from Chimica Mineraria SpA.

In a special embodiment of the method of the invention, it furthercomprises a step in which a preferably non-aqueous electrolyte isdeposited on the electrode layer of a previously producedelectrode/current collector laminate. In said further step theelectrolyte is preferably deposited by a direct or offset gravureprinting operation.

The drying of the electrode paste should preferably be fast in order toobtain a high electrode layer porosity and to allow fast laminatemanufacture. However, the solvent evaporation should not be so fast thatmud cracks and/or delamination appear. Preferably drying is performed ata temperature of 70-160° C., preferably 70-120° C. The drying preferablytakes place in a period of 0.1 to 30 s following the printing operation.

The printing speed can be varied between wide limits. However, theoptimal printing speed is a speed close to the critical speed, i.e. themaximum speed at which proper single-line pick-out takes place.Preferably, during the printing operation, the matrice roll is rotatedat a frequency corresponding to a speed at the roll circumference in therange of 0.01-300 m/min., preferably 5-200 m/min., more preferably50-150 m/min.

In general, the thickness of the coated electrode layer will increasewith increasing cell depth of the applied matrice roll. For a gravureroll, the amount of coating transferred to the web almost alwayscorresponds to approximately 60% of the cell volume (see e.g. Cohen,Edward and Gutoff, Edgar: Modern Coating and Drying Technology, VCH, pp.103-108). Preferably a gravure roll having a cell depth in the range of10-300 μm, more preferably 20-200 μm, even more preferably 50-150 μm, isapplied in the method of the invention.

The pitch number of the applied gravure roll should be sufficiently highto provide for good levelling of the coated paste. On the other hand,the pitch number should be chosen so low that unstable multiple-linepick-out is avoided. Preferably, the pitch number is in the range of4-160 cm⁻¹, more preferably 10-100 cm⁻¹, even more preferably 10-30cm⁻¹.

At a given volume factor (volume of cells per unit surface area), theland area, i.e. the non-graved area of the gravure roll, should bemaximised to sustain proper printing. However, at very high land areas,the levelling process will be hindered. The land to volume ratio shouldbe selected in accordance with the pitch requirements described above.

The cell geometry should allow for the high coating thickness. Angles inthe range of 30-80° are applicable as they allow for fast transfer ofelectrode paste to and from the cells during the printing operation. Thecells are preferably provided by a chemical etching process, whichtypically results in a distribution of cell angles.

Several combinations of blade elasticity, force and contact angle, willallow for printing of the relatively thick coatings. The doctoring blademodulus of elasticity, defined as the ratio of stress (deformationsubject to load) and strain (tendency to resume original shape) shouldbe high.

In general, there exists a relation between the applied force and thecoating thickness; the higher the applied force, the lower the thicknessof the coating. With soft blades, however, the relation may be morecomplex; at medium force, the coating thickness may be increasing withincreasing force, as the tip of the blade is lifted from the gravureroll surface. At low and high forces, the well-known relation ofdecreasing coating thickness with increasing force exists. The bladeloading is increased upon pivoting the base of the blade, i.e. uponincreasing the working angle of the blade. Preferably the working angleshould be relatively low in order to allow for relative thick coatings.

The invention is described in further detail in the drawing, in which:

FIG. 1. shows the viscosity as a function of shear rate (i.e. theviscosity profile) for an electrode paste according to the invention;

FIG. 2. is a schematic drawing of a direct gravure printing operationinvolving a single gravure roll set-up; and

FIG. 3 is a schematic drawing of a direct gravure printing operationinvolving a set-up consisting of five gravure rolls and five dryingrolls.

In FIG. 1 a typical viscosity profile for an electrode paste accordingto the invention is shown. The viscosity of the paste decreases as afunction of shear rate, corresponding to pseudoplastic behaviour.

In FIG. 2 a direct gravure printing operation is shown. The printingapparatus consists of a gravure roll (1) and a flexible, rubber-coveredback-up roll (2). Electrode paste (3) is applied to the gravure roll,followed by scrabing off the excessive amount of electrode paste by adoctor blade (4). A current collector film (5) is introduced into thenip formed by the gravure roll (1) and the back-up roll (2), wherebyelectrode paste is picked out of the cells of the gravure roll to form alayer on the current collector.

In FIG. 3 is shown a complete printing facility for carrying out themethod of the invention. The facility to which a current collector foil(10) is supplied, comprises a means for feeding current collector (20),five gravure rolls (30-34) onto which primer mixture or electrode pasteis applied from the vessels (40-44), and five back-up rolls (50-54).Furthermore, each printing operation is followed by a drying operation,which is carried out using five drying rolls (60-64). Each drying rollis equipped with an exhauster (70-74) for removing evaporated solvents.

In the following examples preparation of primer mixtures according tothe invention is illustrated.

EXAMPLE I

Primer Mixture I

10000 g of CL51 from Wiederhold Siebdruckfarben was diluted with 4050 gof 1-butanol.

EXAMPLE II

Primer Mixture II

4,834 g of Alpeprint H/R (Verschnitt C50) from Nordisk TrykfarveIndustri, Denmark, 1,310 g of a 50:40:10 by weight mixture ofbutylacetate, propylene glycol methyl ether acetate and propylene glycoldiacetate and 357 g of a cationic dispersing agent were mixed. 15 g of asilicone defoaming agent was added. The mixture was stirred in adissolver at 1100 rpm. for 3 min. Under stirring at 1100 rpm., 772 g ofShawinigan Black (100% compressed) was added, and the mixture stirredfor 15 min. at 4000 rpm. Then 1,100 g of a 50:40:10 by weight mixture ofbutylacetate, propylene glycol methyl ether acetate and propylene glycoldiacetate was added. The viscosity of the final mixture was 27 s asmeasured with a Ford Cup no. 4.

EXAMPLE III

Primer Mixture III

6,640 g of Alpeprint H/R (Verschnitt C50) from Nordisk TrykfarveIndustri, Denmark, 1,800 g of a 50:40:10 by weight mixture ofbutylacetate, propyleneglycol methyl ether acetate and propylene glycoldiacetate and 490 g of a cationic dispersing agent were mixed. 20 g of asilicon defoaming agent was added. The mixture was stirred in adissolver at 1100 rpm. for 3 min. Under stirring at 1100 rpm., 1,060 gof Shawinigan Black (100% compressed) was added, and the mixture stirredfor 15 min. at 4000 rpm. 1,900 g of a 50:40:10 by weight mixture ofbutylacetate, propylene glycol methyl ether acetate and propylene glycoldiacetate were added. The viscosity of the final mixture was 38 s asmeasured with a Ford Cup no. 4.

In the following examples preparation of anode pastes according to theinvention is illustrated.

EXAMPLE IV

Anode Paste A

Under stirring, 757 g of a cationic dispersing agent was dissolved in4,300 g of a 35:5:30:30 by weight mixture of odourless petroleum 30,odourless petroleum 60, amyl acetate and propylene glycol methyl etheracetate. 3,846 g of 8% of EPDM (ethylene-propylene-diene-polymethylene)in a 90:10 mixture of odourless petroleum 30:odourless petroleum 60 wasadded without stirring. The resulting mixture was stirred in a dissolverat 1100 rpm. Then 61 g of a mineral oil defoaming agent was added duringa period of 4 min followed by 9 min. of stirring. 3,990 g of coke (LonzaR-LIBA-A) was added, and the stirring continued for 8 min. 1,840 g oflampblack (Degussa Lampblack 101) was added, and the stirring continuedat 1100 rpm for 8 min. Then the stirring speed was increased to 4000 rpmand kept at this value for 10 min. The stirring was stopped, and 200 gof a 50:5:15:30 by weight mixture of odourless petroleum 30, odourlesspetroleum 60, propylene glycol diacetate and propylene glycol methylether acetate was added. Under stirring at 1100 rpm. another 1,509 g ofthe 50:5:15:30 by weight mixture of odourless petroleum 30, odourlesspetroleum 60, propylene glycol diacetate and propylene glycol methylether acetate was added. Following 10 min. of stirring, 16 g of amineral oil defoaming agent was added, along with 700 g of the lattersolvent mixture. The viscosity of the resulting mixture was 41 s asmeasured by a Ford Cup no. 4. Finally, another 250 g of the lattersolvent mixture was added. After 120 h of storage, a viscosity of 30 swas measured using a Ford Cup no. 4.

Before printing, 275 g of odourless petroleum 30 and 275 g of amylacetate, and finally 100 g of a 35:5:30:30 by weight mixture ofodourless petroleum 30, odourless petroleum 60, amyl acetate andpropylene glycol methyl ether acetate was added to the paste.

EXAMPLE V

Anode Paste B

Under stirring, 645 g of a cationic dispersing agent was dissolved in3,690 g of a 35:5:30:30 by weight mixture of odourless petroleum 30,odourless petroleum 60, amyl acetate and propylene glycol methyl etheracetate. 3,300 g of 8% of EPDM (ethylene-propylene-diene-polymethylene)in a 90:10 mixture of odourless petroleum 30:odourless petroleum 60 wasadded without stirring. Then the mixture was stirred in a dissolver at1100 rpm. for 3 min. 45 g of a mineral oil defoaming agent was added,and the stirring continued for 7 min. Then 3,440 g of coke (LonzaR-LIBA-A) was added followed by 10 min. of stirring. 1,590 g oflampblack (Degussa Lampblack 101) was added, and the stirring continuedat 1100 rpm. for 2 min. The stirring speed was then increased to 5000rpm and kept at this value for 9 min. The stirring was stopped, andfollowing 10 min. of storage, 1,400 g of a 50:5:15:30 mixture by weightof odourless petroleum 30, odourless petroleum 60, propylene glycoldiacetate and propylene glycol methyl ether acetate was added. Theviscosity of the resulting mixture was 39 s as measured with a Ford Cupno. 4. Finally, another 200 g of the latter solvent mixture was added.After 120 h of storage, the viscosity was 35 s as measured with a FordCup no. 4.

Before printing, 275 g of odourless petroleum 30 and 275 g of amylacetate and finally 100 g of a 35:5:30:30 by weight mixture of odourlesspetroleum 30, odourless petroleum 60, amyl acetate and propylene glycolmethyl ether acetate was added to the paste.

EXAMPLE VI

Anode Paste C

Under stirring, 645 g of a cationic dispersing agent was dissolved in3,690 g of a 35:5:30:30 by weight mixture of odourless petroleum 30,odourless petroleum 60, amyl acetate and propylene glycol methyl etheracetate. 3,300 g of 8% of EPDM (ethylene-propylene-diene-polymethylene)in a 90:10 mixture of odourless petroleum 30:odourless petroleum 60 wasadded without stirring. The obtained mixture was stirred in a dissolverat 1100 rpm. for 3 min. 45 g of a mineral oil defoaming agent was added,and the stirring was continued for 7 min. Then, 3,440 g of coke (LonzaR-LIBA-A) was added, and the stirring continued for 10 min. 1,590 g oflampblack (Degussa Lampblack 101) was added, and the stirring continuedat 1100 rpm. for 2 min. Then the stirring speed was increased to 5000rpm and kept at this value for 9 min. The stirring was stopped, andfollowing 10 min. of storage, 1,500 g of a 50:5:15:30 mixture ofodourless petroleum 30, odourless petroleum 60, propylene glycoldiacetate and propylene glycol methyl ether acetate was added. Theviscosity of the resulting mixture was 55 s as measured with a Ford Cupno. 4. Finally, another 200 g of the latter solvent mixture was added.After 120 h of storage, the viscosity was 40 s as measured with a FordCup no. 4.

Before printing, 275 g of odourless petroleum 30 and 275 g of amylacetate and finally 100 g of a 35:5:30:30 by weight mixture of odourlesspetroleum 30, odourless petroleum 60, amyl acetate and propylene glycolmethyl ether acetate was added to the paste.

In the following examples preparation of a cathode paste according tothe invention is illustrated.

EXAMPLE VII

Cathode Paste D

Under stirring at 1100 rpm. in a dissolver, 1,580 g of cationicdispersing agent was dissolved in 7,620 g of a 35:5:55:5 by weightmixture of odourless petroleum 30, odourless petroleum 60, butyl acetateand propylene glycol methyl ether acetate. The stirring was stopped, and9,080 g of lithium manganse oxide spinel was added. Stirring wasrestarted at 1100 rpm., and 1,720 g of Shawinigan Black (100%compressed) was added. Stirring at 1100 rpm. was continued for 5 min.Eventually, the mixture was milled for 60 min. in a DYNO-MILL KDL-pilotA from Bachofen, Switzerland, which was connected to the dissolver forrecirculation, and operated at a counter pressure of 0.4 bar.

9,660 g of the above mixture was mixed with 4,170 g of 8% of EPDM inodourless petroleum 30, and stirred at 1100 rpm. During stirring, 30 gof defoaming agent was added, the stirring being continued for 5 min.550 g of a 45:5:45:5 mixture by weight of odourless petroleum 30,odourless petroleum 60, butyl acetate and propylene glycol methyl etheracetate was added. The viscosity of the final mixture was 45 s asmeasured with a Ford Cup no. 4.

In the following examples production of electrode/current collectorlaminates according to the invention is illustrated.

EXAMPLE VIII

A Production Process Involving a Single Gravure Roll

An electrode/current collector laminate was produced by direct gravureprinting of anode paste A using a single gravure roll (cf. FIG. 2)having a circumference of 543 mm. The width of the gravure roll was 800mm, the central 400 mm of which constituted the engraved part. It had anetched pattern displaying a pitch of 14 cm⁻¹, the cell depth being 120μm.

The roll was used for printing of an anode paste onto a 600 mm widealuminium current collector foil onto which a thin protective primer hadbeen deposited in a previous process. The employed printing speed was 17m/min. The working angles of the blade was kept in the range of 45-50°.

Drying was performed by subjecting the laminate to a temperature of 132°C.

The final laminate showed good adhesion between the current collectorand the electrode. The thickness of the electrode layer was 30 μm andthe porosity was approximately 73%, as calculated from weight, thicknessand theoretical compact density.

EXAMPLE IX

A Production Process Involving Four Gravure Rolls

An electrode/current collector laminate was produced by direct gravureprinting using a gravure roll set-up of four rolls (designated gravureroll 1 to 4), each having a circumference of 543 mm and a width of 800mm. The width of the engraved central part of gravure roll 1 was 410 mm,whereas the engraved width of gravure rolls 2, 3 and 4 was 400 mm.Gravure roll 1 had an engraved pattern displaying a pitch of 70 cm⁻¹,whereas the gravure rolls 2, 3 and 4 had an etched pattern displaying apitch of 14 cm⁻¹. The cell depth of gravure roll 1 was 20 μm, whereasthe cell depth of gravure rolls 2 to 4 was 120 μm.

The gravure rolls 1 to 4 of the set-up were used for printing of primermixture I, anode paste A, anode paste B and anode paste C, respectively,using a printing speed of 17 m/min. The working angles of the blades ofall four rolls were in the range of 45-50°.

Drying was performed by drying rolls 1 to 4, each of the gravure rolls 1to 4 being followed by a drying roll of the corresponding number. Theoperational temperatures of the drying rolls 1 to 4 was 137° C., 146°C., 147° C. and 148° C., respectively.

A current collector foil consisting of a 25 μm thick and 600 mm widealuminium foil was fed to the set-up at gravure roll 1 and a finallaminate having a total thickness of 120 μm and consisting of thealuminium foil containing a primer layer and three electrode layers(each having a thickness of 30 μm, 25 μm and 40 μm, respectively) wasremoved from the set-up at drying roll 4.

The produced laminate showed good adhesion between the current collectorand the electrode. The electrode porosity was approximately 70%,calculated from weight, thickness and theoretical compact density.

EXAMPLE X

A Production Process Involving Five Gravure Rolls

An electrode/current collector laminate was produced by direct gravureprinting using a gravure roll set-up of five rolls (cf. FIG. 3), eachhaving a circumference of 543 mm and a width of 800 mm. The width of theengraved central part of rolls 30 and 31 was 410 mm, whereas theengraved width of rolls 32, 33 and 34 was 400 mm. Roll 30 and 31 had anengraved pattern displaying a pitch of 70 cm⁻¹, whereas the rolls 32, 33and 34 had an etched pattern displaying a pitch of 14 cm⁻¹. The celldepth of gravure rolls 30 and 31 was 20 μm, whereas the cell depth ofgravure rolls 32, 33 and 34 was 120 μm.

The gravure rolls 30 to 34 of the set-up were used for printing ofprimer mixture II and III, and three layers of cathode paste,respectively, using a printing speed of 17 m/min. The working angles ofthe blades of all five rolls were in the range of 45-50°.

Drying was performed by drying rolls 60 to 64, each of the gravure rolls30 to 34 being followed by a drying roll. The operational temperaturesof the drying rolls 60 to 64 was 104° C., 105° C., 126° C., 127° C., and126° C., respectively.

A current collector foil consisting of an aluminium foil was fed to theset-up at gravure roll 30 and the final laminate consisting of thealuminium foil containing two primer layers having a total thickness of3 μm and three cathode layers having a total thickness of 44 μm wasremoved from the set-up at drying roll 64.

The produced laminate showed good adhesion between the current collectorand the electrode. The electrode porosity was 65%, calculated fromweight, thickness and theoretical compact density.

In the following example production of a primer-coated current collectoris illustrated.

EXAMPLE XI

A Production Process Involving Two Gravure Rolls

A current collector coated with protective primer layers on both sideswas produced by direct gravure printing using a gravure roll set-up oftwo rolls (designated roll 1 and 2), each having a circumference of 543mm and a width of 800 mm. The width of the engraved central part of thegravure rolls was 410 mm. The gravure rolls had an engraved patterndisplaying a pitch of 70 cm⁻¹ and a cell depth of 20 μm.

The gravure rolls 1 and 2 were used for printing of primer mixture IIand III, respectively, employing a printing speed of 17 m/min and aworking angle of the blades of both rolls in the range of 45-50°.

Printing of protective primer was done on both sides of a 25 μm thickaluminium current collector foil. The production involved a firstprinting step in which primer mixtures II and III were coated onto afirst side of the current collector, followed by drying, and a secondprinting step in which primer mixtures II and III were coated onto theopposite side of the current collector, followed by drying.

Drying was performed by drying rolls 1 and 2, each of the gravure rollsbeing followed by a drying roll of the corresponding number. The dryingtemperature of both drying rolls was 105° C.

The protective 3 μm thick primer layers of the produced primer-coatedcurrent collector showed good adhesion to the current collector.

In the following example production of electrochemical cells from theelectrode/current collector laminates of the invention is illustrated.

EXAMPLE XII

An electrochemical cell was produced from the anode/current collectorlaminate of Example VIII and the cathode current collector laminate ofExample X. The cell was obtained by sandwiching an electrolyte composedof 1M LiPF₆ in propylene carbonate (PC) between said laminates.

The obtained cell displayed a good cyclability, 60% of its initialcapacity remaining after 60 cycles.

What is claimed is:
 1. A method for the production of anelectrode/current collector laminate for an electrochemical device byforming a layer of an electrode paste on at least one side of a currentcollector foil, said method comprising the steps of: a) loading saidelectrode paste into cells or grooves of a patterned matrice roll; b)transferring said electrode paste onto said current collector foileither directly or indirectly via an offset roll; and c) drying saidelectrode paste to provide a layer of at least 20 μm in thickness;wherein said electrode paste comprises particles of an electrodematerial, solvent and binder, and displays a pseudoplastic viscosityprofile.
 2. A method according to claim 1, wherein said electrode pasteis thixotropic.
 3. A method according to claim 1, wherein said viscosityprofile is adjusted by use of one or more auxiliary materials selectedfrom the group consisting of dispersing agents, defoaming agents andrheological control agents.
 4. A method according to claim 1, whereinsaid matrice roll is an engraved gravure roll and said electrode pasteis transferred onto said current collector foil by a direct gravureprinting operation involving a pair of rolls consisting of said engravedgravure roll and a flexible back-up roll defining a nip therebetween,said current collector foil being introduced into said nip.
 5. A methodaccording to claim 1, wherein said matrice roll is an engraved gravureroll and said electrode paste is transferred onto said current collectorfoil by an offset gravure printing operation involving said gravureroll, an offset roll in contact therewith and a flexible back-up roll,said flexible back-up roll and said offset roll defining a niptherebetween, paste from said gravure roll being transferred to saidoffsets roll and subsequently from said offset roll to said currentcollector foil by introduction of said current collector foil into saidnip.
 6. A method according to claim 1, wherein said electrode paste istransferred onto said current collector foil by a flexoprintingoperation involving said patterned matrice roll, a rubber offset roll incontact therewith and a flexible back-up roll, said rubber roll and saidflexible back-up roll defining a nip therebetween, said paste beingtransferred from said matrice roll to said rubber offset andsubsequently to said current collector foil by introduction of saidcurrent collector foil into said nip.
 7. A method according to claim 1,wherein said electrode paste is coated onto said current collector toobtain an electrode layer thickness of 20-300 μm in said laminatefollowing drying.
 8. A method according to claim 1, wherein saidelectrode paste has a porosity of 20-80% in said laminate followingdrying.
 9. A method according to claim 1, wherein said electrode pastehas a porosity of 30-70% in said laminate following drying.
 10. A methodaccording to claim 1, wherein said current collector foil is a metalfoil.
 11. A method according to claim 10, wherein said current collectorfoil is selected from the group consisting of aluminum, copper, nickeland stainless steel foils.
 12. A method according to claim 1, whereinsaid current collector foil is coated with a conductive primer prior tocoating of said electrode paste.
 13. A method according to claim 1,wherein said electrode paste comprises: a) 10-60% by weight of electrodematerial; b) 0-20% by weight of an electronically conductive additive;c) 1-20% by weight of binder; d) 20-88% by weight of solvent; and e)0-20% by weight of auxiliary materials.
 14. A method according to claim13, wherein said electrode paste comprises: a) 25-50% by weight ofelectrode material; b) 2-10% by weight of an electronically conductiveadditive; c) 1-10% by weight of binder; d) 30-70% by weight of solvent;and e) 1-10% by weight of auxiliary materials.
 15. A method according toclaim 13, wherein said auxiliary materials are selected from the groupconsisting of dispersing agents, defoaming agents and rheologicalcontrol agents.
 16. A method according to claim 13, which comprises aplurality of electrode coating operations each followed by drying ofprinted electrode paste and in which at least one of said printingoperations provides a layer at least 20 μm thickness.
 17. A methodaccording to claim 1, wherein said matrice roll is a gravure roll havinga cell depth in the range of 100-300 μm.
 18. A method according to claim17, wherein said cell depth is in the range 120-200 μm.
 19. A methodaccording to claim 1, wherein said matrice roll is a gravure roll havinga pitch number in the range of 4-20 cm⁻¹.
 20. A method according toclaim 19, wherein said pitch number is in the range 4-15 cm⁻¹.
 21. Amethod for the production of an electrode/current collector laminate foran electrochemical device by forming a layer of an electrode paste on atleast one side of a current collector foil, said method comprising thesteps of: a) loading said electrode paste into cells or grooves of apatterned matrice roll; b) transferring said electrode paste onto saidcurrent collector foil either directly or indirectly via an offset roll;and c) drying said electrode paste to provide a layer of at least 20 μmin thickness; wherein said electrode paste comprises particles of anelectrode material, solvent and binder, and displays a pseudoplasticviscosity profile and wherein said current collector foil is aconductive polymer foil.
 22. A method for the production of anelectrode/current collector laminate for an electrochemical device byforming a layer of an electrode paste on at least one side of a currentcollector foil, said method comprising the steps of: a) loading saidelectrode paste into cells or grooves of a patterned matrice roll,wherein said electrode paste comprises: i) 10-60% by weight of electrodematerial, ii) 0-20% by weight of an electronically conductive additive,iii) 1-20% by weight of binder, iv) 20-88% by weight of solvent, and v)0-20% by weight of auxiliary materials; b) transferring said electrodepaste onto said current collector foil either directly or indirectly viaan offset roll; c) drying said electrode paste to provide a layer of atleast 20 μm in thickness wherein said electrode paste comprisesparticles of an electrode material, solvent and binder, and displays apseudoplastic viscosity profile; and d) performing a plurality of pastetransferring operations to said current collector foil each followed bydrying the electrode paste and in which at least one of saidtransferring operations provides a layer at least 20 μm in thicknesswherein in at least two of said paste transferring steps the pastediffers in porosity so as to obtain a porosity profile in said electrodewhich varies with depth.